1. Field of the Invention
The invention relates to a method for interrupting power to an inductive load in a three-phase high voltage network by means of a vacuum switches having quick dielectric recovery characteristics.
2. Prior Art
It has been found that upon interruption of an inductive load in a three-phase high voltage network, sometimes high overvoltages occur on the load to be interrupted. Since these overvoltages may cause considerable damage to the network due to insulation breakdown and the like, several investigations have been made in order to discover the likely causes of these high overvoltages. From these investigations it has become apparent that these high overvoltages occur only when in the pertaining circuit at least one of a number of conditions is fulfilled, the most important ones of which are:
1. Switching should be performed by switches having a quick dielectric recovery; this condition being fulfilled by vacuum switches. PA1 2. The requirement of a predetermined network constellation; this implying inter alia, the presence of a sufficient capacitance between the phases on both the source side and the load side of the switch. PA1 3. The switching time should be such that upon separation of the contact members one pole of the switch is close to a zero current passage.
Upon interruption of a load under the above conditions, in the first phase interrupted at zero current passage an overvoltage will occur that may result in re-ignitions in this phase. Under the prevailing circumstances the current resulting from these re-ignitions and having a very high frequency will be entirely or partially superimposed on the network current in both the other phases, which although already being interrupted still carry current. When the resultant of the superimposed current and the network current in these phases becomes about zero both the other phases will also be interrupted. Since, however, at the re-ignition moment the network current in the first phase causing the re-ignitions is close to a zero current passage, the momentary current value in both the other phases is relatively high, this resulting in a so-called high current chopping. In the literature this chopping of the relatively high current is generally called "virtual chopping". In view of the fact that with this "virtual chopping" the di/dt-value is high, very high overvoltages may occur in the installation during "virtual chopping".
In a publication by M. Murano et al, "Three-phase Simultaneous Interruption in Interrupting Inductive Current Using Vacuum Switches", I.E.E.E. Transactions on Power Apparatus Systems, January/Feburary 1974, pages 272-280, concerning the phenomenon "virtual chopping" it has been proposed to include a resistance in series with a capacitance in parallel to the load between the phases and ground. This, however, entails the drawback that the resistance and capacitance values have to be adapted to the pertaining circuit and the load to be interrupted, this solution moreover being rather expensive in case of high currents.
Another method referred to in the above publication employs a non-linear resistance the high cost of which is likewise of disadvantage.
A further method employs surge suppressors in order to restrict the detrimental results of "virtual chopping". This method, however, also entails the drawback of relatively high expenses accompanied by the fact that the surge suppressors have to be installed as close as possible to the load.
The above mentioned methods have furthermore in common that they are only operative under those network conditions for which they have been designed. Upon a change in these conditions one shall therefore have to adapt the method to this change. Furthermore the abovementioned methods have in common that they have not been directed to the prevention of "virtual chopping" but only to a protection against the results thereof.
A better method will therefore be complete prevention of "virtual chopping". This may be achieved by the provision of not fulfilling one of the abovementioned three conditions for the occurrence of "virtual chopping".
If one wishes to profit by the specific advantages of a vacuum switch the first condition will is inherently met. For eliminating the second condition the conductors connected to both the source side and the load side of the switch have to be completely shielded with respect to each other. In practice, however, such appears to be a difficult and expensive task.
Finally the occurrence of condition three may be obviated by synchronizing the switching time with respect to the zero current passage in such a manner that the separation of the contact members does not happen close to zero current passage. However, such a technique incurs the use of complicated and expensive devices.